Zdeněk Mašín

Shape and core excited resonances in electron collisions with diazines

We present a comprehensive ab-initio study of electron collisions with pyrazine, pyrimidine, and pyridazine. The emphasis is placed on the identification and characterization of electron resonances in these systems. We use the R-matrix method and show that analysing the time-delay reveals resonances whose signature is not visible in the eigenphase sums. In addition to the well known π∗ resonances below 5 eV, we find three core-excited shape resonances in the energy range 5.5-8.5 eV and a few Feshbach resonances in the dipolar molecules. Additionally, 11 resonances with little effect on the elastic scattering from ground state diazines (but significant effect in elastic collisions with the molecules in an excited state) are found and characterized. We correlate these resonances across the three molecules and discuss their possible correspondence to resonances described in earlier studies on uracil.

Elastic and inelastic low-energy electron collisions with pyrazine.

We present results of ab-initio scattering calculations for electron collisions with pyrazine using the R-matrix method, carried out at various levels of approximation. We confirm the existing experimental and theoretical understanding of the three well-known π∗ shape resonances. In addition, we find numerous core-excited resonances (above 4.8 eV) and identify their most likely parent states. We also present differential cross sections, showing high sensitivity to the scattering model chosen at low energies. We make recommendations regarding the selection of models for scattering calculations with this type of targets.

Electron scattering cross section calculations for polar molecules over a broad energy range

We report computational integral and differential cross sections for electron scattering by two different polar molecules, HCN and pyrimidine, over a broad energy range. We employ, for low energies, either the single-centre expansion (ePOLYSCAT) or the R-matrix method, while for the higher energies we select a corrected form of the independent-atom representation (IAM-SCAR). We provide complete sets of integral electron scattering cross sections from low energies up to 10,000 eV. Our present calculated data agree well with prior experimental results.

Electron scattering from pyridine

We have calculated cross sections for elastic and inelastic electron scattering from pyridine in the energy range 1 eV to 1 keV. The R-matrix and IAM-SCAR methods have been used for low and higher collision energies, respectively. Agreement with available theoretical data is good. We have also examined the formation of shape resonances and compared our results with existing experimental data. We compare the results with data for electron scattering from pyrimidine.

The role of multichannel effects in the photoionization of the NO 2 molecule: an ab initio R- matrix study

We present the first ab initio photoionization calculations for the NO 2 molecule in its equilibrium geometry using the multichannel R- matrix method and a multiconfigurational description of the system. We focus on the role of correlation in NO 2 photoionization and find that it plays a key role, both at the level of partial cross sections and asymmetry parameters. For the most sophisticated model used here, we achieve excellent agreement with the experimental data of Baltzer et al (2009 Chem. Phys. 237 451–70 ) for the asymmetry parameters of angle-resolved photo-electron spectra. We also present and analyse the angle-resolved photoionization dipoles for photon energies up to 90 eV and for the two lowest-energy ionization channels. Our results should advance the analysis of experiments in the field of attosecond spectroscopy, especially high harmonic generation, where angle-resolved photorecombination dipoles become crucial for the interpretation of experiments, even for randomly oriented molecular ensembles, due to coherent addition of signals from different orientations.

Absolute cross sections for electronic excitation of pyrimidine by electron impact.

We measured differential cross sections for electron-impact electronic excitation of pyrimidine, both as a function of electron energy up to 18 eV, and of scattering angle up to 180°. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. The differential cross sections were summed to obtain integral cross sections. These are compared to results of R-matrix calculations, which successfully reproduce both the magnitude of the cross section and the major resonant features. Comparison of the experiment to the calculated contributions of different symmetries to the integral cross section permitted assignment of several features to specific core-excited resonances. Comparison of the resonant structure of pyrimidine with that of benzene revealed pronounced similarities and thus a dominant role of π-π(∗) excited states and resonances. Electron energy loss spectra were measured as a preparation for the cross section measurements and vibrational structure was observed for some of the triplet states. A detailed analysis of the electronic excited states of pyrimidine is also presented.

Resonance effects in elastic cross sections for electron scattering on pyrimidine: Experiment and theory

We measured differential cross sections for elastic (rotationally integrated) electron scattering on pyrimidine, both as a function of angle up to 180(∘) at electron energies of 1, 5, 10, and 20 eV and as a function of electron energy in the range 0.1-14 eV. The experimental results are compared to the results of the fixed-nuclei Schwinger variational and R-matrix theoretical methods, which reproduce satisfactorily the magnitudes and shapes of the experimental cross sections. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. Resonant structures are observed at 0.2, 0.7, and 4.35 eV and calculations for different symmetries confirm their assignment as the X̃(2)A2, Ã(2)B1, and B̃(2)B1 shape resonances. As a consequence of superposition of coherent resonant amplitudes with background scattering the B̃(2)B1 shape resonance appears as a peak, a dip, or a step function in the cross sections recorded as a function of energy at different scattering angles and this effect is satisfactorily reproduced by theory. The dip and peak contributions at different scattering angles partially compensate, making the resonance nearly invisible in the integral cross section. Vibrationally integrated cross sections were also measured at 1, 5, 10 and 20 eV and the question of whether the fixed-nuclei cross sections should be compared to vibrationally elastic or vibrationally integrated cross section is discussed.

Recent progress in electron scattering from atoms and molecules

We present and discuss recent results, both experimental and theoretical (where possible), for electron impact excitation of the 3s[32]1 and 3s′[12]1 electronic states in neon, elastic electron scattering from the structurally similar molecules benzene, pyrazine, and 1,4-dioxane and excitation of the electronic states of the important bio-molecule analogue α-tetrahydrofurfuryl alcohol. While comparison between theoretical and experimental results suggests that benchmarked cross sections for electron scattering from atoms is feasible in the near-term, significant further theoretical development for electron-molecule collisions, particularly in respect to discrete excitation processes, is still required.We present and discuss recent results, both experimental and theoretical (where possible), for electron impact excitation of the 3s[32]1 and 3s′[12]1 electronic states in neon, elastic electron scattering from the structurally similar molecules benzene, pyrazine, and 1,4-dioxane and excitation of the electronic states of the important bio-molecule analogue α-tetrahydrofurfuryl alcohol. While comparison between theoretical and experimental results suggests that benchmarked cross sections for electron scattering from atoms is feasible in the near-term, significant further theoretical development for electron-molecule collisions, particularly in respect to discrete excitation processes, is still required.

Electron attachment to molecular clusters

We study experimentally and theoretically the effect of hydrogen bonding in the formation of molecular temporary negative ions. A new experimental set up enables us, for the first time, to perform electron scattering experiments with selected clusters. This selectivity facilitates the comparison with theoretical results.

Electron correlations and pre-collision in the re-collision picture of high harmonic generation

We discuss the seminal three-step model and the recollision picture in the context of high harmonic generation in molecules. In particular, we stress the importance of multi-electron correlation during the first and the third of the three steps of the process: (1) the strong field ionization and (3) the recombination. We point out how accurate account for multi-electron correlations during the third, recombination, step allows one to gauge the importance of pre-collision: the term coined by J. H. Eberly to describe unusual pathways during the first, ionization, step.